Field of the Invention
This invention relates to a lifting device of an industrial truck with a load handling arrangement located on a lifting frame so that the load handling arrangement can be raised and lowered, in which, to raise and lower the load handling arrangement, a hydraulic lift cylinder device is provided that can be actuated by an electrically actuatable control valve device, in which an electronic control device that controls the control valve device is provided.
The invention further relates to a method for setting down a load carried on a load handling arrangement of an industrial truck on a surface, in which the load handling arrangement is located so that the load handling arrangement can be raised and lowered on a lifting frame of the industrial truck, and, to raise and lower the load handling arrangement, a hydraulic lift cylinder device is provided that can be actuated by an electrically actuatable control valve device, in which the control valve device is actuated into a lowered position as a function of the deflection of a control element.
Description of Related Art
Industrial trucks are used for the handling of loads with load handling means that are formed by a load carriage that can be raised and lowered on the lifting frame and an attachment that is fastened to it. The attachment can be, for example, a load fork consisting of fork tips which can be run underneath a load such as a pallet. The attachment can also be a clamping device, such as a baling or roll clamp with which a load is held by lateral clamping. Flexible traction means such as a load chain, for example, are fastened on the load carriage to raise and lower the load handling means and are fastened by the first end to the load carriage, guided around a return pulley and fastened by a second end to the lifting frame. The traction means are actuated by a lift cylinder device, which generally includes one or more lift cylinders, each with a telescoping piston rod.
As the load handling means are lowered during a lowering process, under some operating conditions the descending movement of the load handling means can be stopped and the piston rod of the lift cylinder device can be retracted further. In this case, the traction means go slack, so that, after the load handling means are released, there is a sudden and jerky descending movement of the load handling means until the traction means are once again taut. This can cause damage to the load being handled and/or an overloading of the traction means.
Operating conditions of this type can occur, for example, when a load is being set down by load handling means in the form of a load fork on a shelf compartment if the load tips of the load fork, as the load is being set down in the shelf compartment, are in contact with the shelf compartment when the lift cylinder device is driven in further and, therefore, the traction means are slack and lose tension. As soon as the load tips become free again as they are pulled out of the shelf compartment, there is an undesirable sudden descending movement of the load fork, until the traction means are once again taut.
An additional operating situation of this type occurs if, during the lowering of the load by load handling means in the form of a clamp device, the load held by the clamp device is sitting on the floor or in a shelf compartment and is in contact with the floor or the shelf compartment. If the descending movement of the lift cylinder device continues to be actuated, the clamping device is held in position by the load, so that the traction means connected with the load carriage as a result of the further driving-in motion of the piston rod of the lift cylinder device go slack and lose tension. If the clamping device is then opened, the clamping device fastened to the load carriage descends suddenly until the traction means are again taut. This can result in damage to the load as well as an overloading of the traction means.
To prevent the sudden uncontrolled descent of the load handling means when the traction means go slack, it is already known that a hydraulic load protection function can be provided.
This hydraulic load protection function includes a dump valve in the form of a mechanical check valve, which is located in a connecting line between the control valve device and the lift cylinder device parallel to a check valve that opens toward the lift cylinder device. The dump valve is actuated by a spring into a closed position and is held open by pressure present in the connecting line. As soon as, during a lowering process, the pressure present in the connecting line is less than the spring bias of the spring, which acts as a pressure threshold, e.g. during the un-loading of the load handling means, the dump valve is actuated into the closed position and the connecting line is shut off so that the descending movement and thus the lowering of the load handling means stops abruptly. The purpose of this hydraulic load protection function is to prevent the traction means from going slack while setting down a load on the intended surface with a heavy attachment. A lifting device with a dump valve of this type in the connecting line to keep the traction means taut is illustrated in FIG. 6 in U.S. Pat. No. 4,955,461 A1. The pressure threshold of the dump valve, which is mechanically specified and set by the spring, equals the pressure in the connecting line that occurs when the load handling means are not loaded. With an attachment in the form of a baling or roller clamp as the load handling means, with which rolls of paper are handled, a dump valve of this type can prevent the destruction of the rolls of paper being carried when the roll of paper is set down in the designated location.
However, this load protection function can be realized with a dump valve only if a sufficiently heavy attachment is present to handle the load as load handling means, which, in the raised position, generates a sufficiently high pressure in the connecting line, for example if the weight of the attachment is greater than 500 kg, and if, when the load is being set down, it is clamped shut and therefore the load is held in place by clamps, so that a pressure of approximately 0 bars results in the connecting line when the load is set down on the designated surface. This occurs because the attachment must generate a sufficiently high pressure to place the pressure threshold of the dump valve far from the pressure that occurs during the raising of the lifting device without the attachment, i.e. the pressure during the raising of the elevatable parts of the mast of the lifting frame, to prevent a lowering of the attachment even at low pressures with an actuation of the dump valve connected with an abrupt stop in the downward movement during operation.
DE 10 2012 101 734 A1 describes a lifting device of an industrial truck, with which the load protection function described above that prevents the traction means from going slack when a load is set down on a designated surface, is achieved electronically. The pressure in the connecting line between the control valve device and the lift cylinder device is measured by means of a pressure sensor and, during the lowering of the load handling means, the control valve device is actuated into a closed position to stop the descent if the pressure in the connecting line is below a pressure limit value. The pressure limiting value is slightly below the pressure that corresponds to the empty load handling means, i.e. when no load is present. The pressure limiting value is therefore selected so that it is slightly lower than the pressure that occurs during the lowering of the load handling means without a load. With a pressure limit value of this type, the insertion movement of the lift cylinder device is terminated by a corresponding actuation of the control valve device into the closed position and the traction means are prevented from going slack, as soon as the load handling means, or a load clamped by the load handling means, are in contact with the surface on which they are to be set down and the load handling means have terminated their descending movement.
One disadvantage of lifting devices of this type, however, is that when a load is being carried freely, for example a pallet being supported by a load fork as the load handling means, the descending movement is stopped only if the load handling means are in contact with the surface on which the load is to be deposited, i.e. the load handling means have terminated their descending movement. If an operator wants to drive away with the load handling means from the surface on which the load was deposited, the operator must therefore raise the load handling means slightly from the surface on which the load was deposited and, therefore, readjust the load handling means after the load has been set down on the surface where it was deposited to prevent the load handling means from scraping the surface as the industrial truck is driven away. The readjustment of the height of the load handling means, however, requires a complex maneuver on the part of the operator as the load is being set down on the designated surface and can result in the scraping of the load handling means on the surface as the industrial truck is driven away from the surface where the load has been set down, with corresponding wear of the load handling means.